Device and method for epitaxially growing gallium nitride layers

ABSTRACT

An epitaxial growth system comprises a housing around an epitaxial growth chamber. A substrate support is located within the growth chamber. A gallium source introduces gallium into the growth chamber and directs the gallium towards the substrate. An activated nitrogen source introduces activated nitrogen into the growth chamber and directs the activated nitrogen towards the substrate. The activated nitrogen comprises ionic nitrogen species and atomic nitrogen species. An external magnet and/or an exit aperture control the amount of atomic nitrogen species and ionic nitrogen species reaching the substrate.

This application is a division of Ser. No. 08/371,708, filed Jan. 13,1995, which is a continuation-in part of Ser. No. 08/113,964, filed Aug.30, 1993, U.S. Pat. No. 5,385,862, which is a continuation ofapplication 07/670,692, filed as PCT/US92/02242 Mar. 18, 1992, publishedas WO92/16966 Oct. 1, 1992, entitled "DEVICE AND METHOD FOR EPITAXIALLYGROWING GALLIUM NITRIDE LAYERS", which was abandoned.

FIELD OF THE INVENTION

The invention relates to light emitting devices, and specifically tolight emitting III-V nitride devices. The invention further relates to amethod of preparing monocrystalline gallium nitride thin films byelectron cyclotron resonance microwave plasma assisted molecular beamepitaxy (ECR-assisted MBE). The invention also relates to a method forthe preparation of n-type and p-type gallium nitride (GaN) rims and amethod for forming p-n junctions from these films.

BACKGROUND OF THE INVENTION

Efforts have been made to prepare monocrystalline GaN because of itspotentially useful electrical and optical properties. GaN is a potentialsource of inexpensive and compact solid-state blue lasers. The band gapfor GaN is approximately 3.4 eV, which means that it can emit light onthe edge of the UV-visible region.

Despite the desirability of a monocrystalline GaN layer, its developmenthas been hampered by the many problems encountered during the growthprocess. Previous attempts to prepare monocrystalline GaN films haveresulted in n-type films with high carder concentration. The n-typecharacteristic is attributed to nitrogen vacancies in the crystalstructure which are incorporated into the lattice during growth of thefilm. Hence, the film is unintentionally doped with nitrogen vacanciesduring growth. Nitrogen vacancies affect the electrical and opticalproperties of the film. ECR-assisted metalorganic vapor phase epitaxygave GaN films that were highly conductive and unintentionally dopedn-type (S. Zembutsu and T. Sasaki J. Cryst. Growth 77, 25-26 (1986)).Carder concentrations and mobilities were in the range of 1×10¹⁹ cm⁻³and 50-100 cm² V⁻¹ s⁻¹. Efforts to dope the film p-type were notsuccessful. The carrier concentration was reduced by compensation, thatis, the effect of a donor impurity is "neutralized" by the addition ofan acceptor impurity.

Highly resistive films were prepared by sputtering using an ultra-puregallium target in a nitrogen atmosphere. The films were characterizedn-type and the high resistivity was attributed to the polycrystallinenature of the films (E. Lakshmi, et al. Thin Solid Films 74, 77 (1977)).

In reactive ion molecular beam epitaxy, gallium was supplied from astandard effusion cell and nitrogen was supplied by way of an ionizedbeam. Monocrystalline films were characterized n-type, but higherresistivities of 10⁶ ω-cm and relatively low carder concentrations andmobilities (10¹⁴ cm⁻³ 1-10 cm² V⁻¹ s⁻¹, respectively) were obtained (R.C. Powell, et al. in "Diamond, Silicon Carbide and Related Wide BandgapSemiconductors" Vol. 162, edited by J. T. Glass, R. Messier and N.Fujimori (Material Research Society, Pittsburgh, 1990) pp.525-530).

The only reported p-type GaN was a Mg-doped GaN treated after growthwith low energy electron beam irradiation (LEEBI). P-type conduction wasaccomplished by compensation of n-type GaN (H. Amano et al. Jap. J ApplPhys. 28(12), L2112-L2114 (1989)).

Current methods of preparing GaN do not permit control of nitrogenvacancies within the lattice. Thus it has not been possible to prepareintrinsic GaN. Additionally, it is desirable to control the dopingprocess in GaN films, thereby enabling the production of p-n junctions.The present invention presents a method to prepare near-intrinsicmonocrystalline GaN films and to selectively dope these films n- orp-type.

SUMMARY OF THE INVENTION

A method according to this invention for preparing highly insulatingnear-intrinsic monocrystalline GaN films uses ECR-assisted MBE. In apreferred embodiment, a molecular beam source of gallium (Ga) and anactivated nitrogen (N) source is provided within an MBE growth chamber.The desired substrate is exposed to Ga and activated nitrogen. A film isepitaxially grown in a two step process comprising a low temperaturenucleation step and a high temperature growth step. The nucleation steppreferably occurs by exposure of the substrate to gallium and nitrogenplasma at a temperature preferably in the range of 100°-400° C., buttemperatures up to 550° may also be used. The high temperature growthstep is preferably carried out in the temperature range of 600°-900° C.Preferred substrates include, but are not limited to, (100) and (111)silicon; (0001) (basal plane), (11-20) (a-plane), and (1-102) (r-plane)sapphire; (111) and (100) gallium arsenide; magnesium oxide; zinc oxide;and (6 H-) and (3 C-) silicon carbide. The preferred source of activatednitrogen species is a nitrogen plasma which can be generated by electroncyclotron resonance microwave plasma or a hot tungsten filament or otherconventional methods.

In a preferred embodiment, the nitrogen plasma pressure and Ga fluxpressure are controlled, thus preventing the beading of metallic galliumon the film surface and the forming of nitrogen vacancies within thelattice. The Ga flux is preferably in the range of 2.0-5.0×10⁻⁷ torr.There is preferably an overpressure of nitrogen in the growth chamber,more preferably in the range of 10⁻³ -10⁻⁵ torr.

In yet another preferred embodiment, the low temperature nucleation stepincludes exposure of the substrate to Ga and nitrogen for a period oftime in the range of 3-15 minutes. A film with a thickness of 200-500 Åis deposited which is amorphous or crystalline with defects (having alarge number of misoriented domains and stacking faults) at the lowtemperature of the nucleation step. Also, a film of about 100-1000 Å mayalso be used. The nucleation step film can be further crystallized byheating at 600°-900° C. C. in the presence of activated nitrogen.Subsequent growth at higher temperatures, preferably 600°-900° C,results in the epitaxial growth of monocrystalline near-intrinsic GaNfilm. Preferred thickness of the growth layer is in the range of0.5-10μm.

In another aspect of this invention, the monocrystalline GaN film ispreferentially doped n- or p-type. To generate a p-type semiconductor,the MBE growth chamber is equipped with Ca, activated nitrogen, andacceptor sources. Acceptor sources include Group II elements such as Be,Zn, Cd, Mg, and Ca. The substrate is bombarded with electrons either byapplying a positive bias to the substrate surface or a metal grid placeddirectly in front of the substrate. Conditions for low and hightemperature deposition are as described above. Exposing the substrate toCa, nitrogen and acceptor sources results in a doped GaN film, wherebythe acceptor takes on an electron and is incorporated into the latticeas a negatively charged species. A charged acceptor species requiresless energy to incorporate into the GaN lattice than a neutral acceptor.To dope the material n-type the substrate is bombarded with positiveions by biasing either the substrate or the grid negatively. Thus, thedonor impurities incorporate into the GaN in their charged state. Thisrequires less energy than to incorporate a neutral donor species.Suitable donors include Groups IV and VI elements.

Practice of this invention affords near-intrinsic GaN films withresistivities of up to 10¹⁰ ohms-cm and mobilities of 100 cm² V⁻¹ s⁻¹ at200° C. At room temperature, mobilities of up to 1000 cm² V⁻¹ s⁻¹ havebeen measured by photoconductivity measurements. P-type and n-typesemiconductors can be selectively prepared simply by choice of surfaceor grid bias and impurity source. It is possible to efficientlymanufacture p-n junctions using the methods of this invention.

According to another aspect of the invention group III-V nitrides can beused for fabrication of short wavelength light emitting devices anddetectors due to the capability of obtaining a wide spectral region ofdirect band-to-band transitions (1.9-6.2 eV) as we as higher devicestability than II-VI based devices.

According to another embodiment of the present invention, a p-n junctiondevice comprises a substrate, an Ga-nitride buffer layer formed on thesubstrate, a first doped GaN layer formed on the GaN buffer layer, asecond doped GaN layer doped oppositely of the first doped GaN layerformed on at least a portion of the first doped GaN layer. A firstelectrode contacting the first doped GaN layer and a second electrodecontacting the second doped GaN layer are formed. This device provides apeak emission of about 430 nm.

According to a further aspect of the present invention a method offorming a p-n junction device comprises the steps of providing ahydrogen free enclosure in which a single crystalline substrate may beprovided. Molecular gallium and activated nitrogen are then introducedinto the enclosure at the substrate. The concentration of atomic orionic nitrogen species directed at the substrate is controlled duringgrowth of a GaN layer.

According to yet another aspect of the present invention, an epitaxialgrowth system comprises a housing around an epitaxial growth chamber. Asubstrate support is located within the growth chamber. A gallium sourceintroduces gallium into the growth chamber and directs the galliumtowards the substrate. An activated nitrogen source introduces activatednitrogen into the growth chamber and directs the activated nitrogentowards the substrate. The activated nitrogen comprises ionic nitrogenspecies and atomic nitrogen species. An external magnet and/or an exitaperture control the amount of atomic nitrogen species and ionicnitrogen species reaching the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an ECR-assisted MBE growth chamber.

FIG. 2a is an X-ray diffraction pattern from a GaN fill on (11-20)sapphire grown from a one-step process.

FIG. 2b is an X-ray diffraction pattern from a GaN film on (11-20)sapphire grown from a two-step process.

FIG. 3 is a schematic illustration of a device for doping GaN films.

FIG. 4 is a cross-sectional view of one embodiment of an ECR-assistedMBE growth chamber.

FIG. 5(a) is a schematic depiction of the flow of magnetic field linesemanating from an ECR source having no external solenoid.

FIG. 5(b) is a schematic depiction of the flow of magnetic field linesemanating from an ECR source when an external solenoid is used,according to one aspect of the present invention.

FIG. 6 is a graph showing the effects of variations in the magnet,current for an external solenoid and the relative ion density at thesubstrate.

FIG. 7 is a graph showing the I-V characteristics of a Langmuir probeoperating in a device according to one aspect of the invention.

FIG. 8 is a schematic illustration of an effusion cell exit apertureaccording to one aspect of the present invention.

FIG. 9 is a graph illustrating an optical emission spectra for an ECRnitrogen plasma source.

FIG. 10 is a graph illustrating an optical emission spectra for an ECRnitrogen plasma source operating with an exit aperture according to anembodiment of the present invention.

FIG. 11 is a graph illustrating relative ion emission intensity as afunction of the nitrogen flow rate and aperture size of an ECR deviceaccording to one aspect of the present invention.

FIG. 12 is a p-n junction device according to one embodiment of thepresent invention.

FIGS. 13(a)-(c) depict surface morphologies of films grown according tothe present invention using an ECR source operating at differentmicrowave powers.

FIG. 14 is a p-n junction device according to another embodiment of thepresent invention.

FIG. 15 is a graph showing the EL spectra of a light emitting diodegrown without (a) and with (b) an external solenoid.

FIG. 16 is a graph showing the I-V characteristics of a p-n junctiondevice according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The unintentional doping of GaN has been attributed to the formation ofnitrogen vacancies in the GaN lattice. GaN decomposes (and losesnitrogen) at about 650° C., well below the processing temperatures(>1000° C.) of the MOCVD processes referred to above. Therefore, thegrowth process itself provides sufficient thermal energy for vacancyformation. Growth processes at lower temperatures should reduce thenumber of nitrogen vacancies in the lattice, prevent the unintentionaln-type doping of the GaN lattice and result in intrinsic GaN.

The practice of the present invention forms GaN at significantly lowerprocessing temperatures using an activated nitrogen source. An ECRmicrowave nitrogen plasma is the preferred activated nitrogen source. Atwo step heating process permits the formation of monocrystalline GaN atlower processing temperatures.

One embodiment of an ECR-MBE system used in the present invention isshown in FIG. 1. An ECR-system 10 is integrated with an MBE system 11 byattaching the ECR system 10 to an effusion port 12. ECR system 10 may bean AsTeX model-1000 ECR source, for example. The ECR system 10 includesa microwave generator 13, a waveguide 14, a high vacuum plasma chamber15, and two electromagnets 16 and 17. The microwaves at 2.43 GHz arecreated in the microwave generator 13 and travel down the rectangularwaveguide 14. The microwave power (100-500 W) passes from the waveguide14 into the plasma chamber 15. Nitrogen flows into the plasma chamber 15through a mass flow controller 18. The mass flow controller 18 maintainsan adjustable constant flow rate. The plasma chamber 15 is surrounded bythe two electromagnets 16 and 17. The upper magnet 16 is powered by a 2kW power supply (not shown) and the lower magnet 17 is powered by a 5 kWpower supply (not shown). Positioning of the electromagnets in this wayresults in a more intense stable plasma.

The upper electromagnet 16 sets the free electrons in the chamber 15into cyclotron orbits. The cyclotron frequency is dependent upon thestrength of the magnetic field and the electron charge-to-mass ratio.Since all the electrons assume cyclotron orbits, the energy lost inrandom motion and collisions is reduced. Additionally, the plasma willbe confined to the center of the chamber 15. The magnetic field isadjusted such that the frequency of oscillation of the microwaves isexactly equal to the cyclotron frequency of the electrons. Nitrogen (N₂)is then introduced into the chamber through the mass flow controller 18and part of it is decomposed into atomic and ionic nitrogen and anotherpart of it is convened into excited molecular nitrogen (N₂) by impactwith the high energy electrons. Atomic nitrogen species and excitedmolecular species are known as neutral excited species. The lowerelectromagnet 17 then guides the ions through the effusion port 12towards a substrate 19 which is positioned on and supported by acontinuous azimuthal rotation (C.A.R.) unit 20 in a growth chamber 21 ofthe MBE system 11. The growth chamber 21 is located in a housing 50 intowhich the effusion ports are connected. The C.A.R. 20 can be rotatedbetween 0 and 120 rpm. On certain substrates, GaN films grow in thewurtzite structure and on others in the zincblende structure. Suchsubstrates include for example sapphire (GaN in wurtzitic structure) andSi(100) (GaN in zincblende structure). Gallium flux is generated in aKnudsen effusion cell 22.

In a typical process, the substrate 19 is sputter-etched by the nitrogenplasma at about 600° C., for example. Other high temperatures, fromabout 600° C. to about 900° C., for example, may also be used. Thisprocess effects nitridation. Nitridation is a process in which sapphire(Al₂ O₃) is bombarded with nitrogen at relatively high temperatures. Thenitrogen replaces the oxygen on the surface of the sapphire and createsatomically smooth AlN. After nitridation, the substrate is cooled downto 270° C. in the presence of the nitrogen plasma. A gallium shutter 23is then opened to deposit the initial buffer layer of GaN. The use of anactivated nitrogen source permits the deposition of GaN at this lowtemperature. The buffer layer is allowed to nucleate over ten minutes,for example, and then the gallium shutter 23 is closed to stop thenucleation of the film. The substrate is then brought slowly to 600° C.at the rate of 4° C. every 15 seconds in the presence of the nitrogenplasma. The nitrogen overpressure also helps reduce the formation ofnitrogen vacancies.

Once at 600° C., the substrate 19 is kept at this temperature for 30minutes in the presence of nitrogen plasma to ensure that the GaN bufferlayer crystallizes. The gallium shutter 23 is opened once again to growthe GaN monocrystalline film. The thickness of the film is preferablyabout 1 μm, although in theory there is no limitation to film thickness.Nitrogen pressure and gallium flux are kept constant during the entireprocess.

The two step growth process allows for the nucleation of a buffer layer.The buffer layer is grown at a temperature in the range of 100° C.-400°C. but a range of about 100° C. to about 550° C. may also be used. Up toabout 400° C., the nucleation layer is mostly amorphous. At about 400°C., the layer becomes crystalline with defects due to stacking faultsand misoriented domains. If an amorphous nucleation layer is grown, thelayer should be relatively thin (e.g. about 100-200 Å), but shouldcompletely cover the substrate. On some instances, thinner buffer layersare desirable because subsequent crystallization of a thin amorphousnucleation layer will be more efficient and will not take as much timeas a thicker amorphous layer would. An advantage of using a temperaturethat causes an amorphous layer is that amorphous layers more readilyuniformly cover the entire substrate.

At higher temperatures, such as between about 400° C. and about 550° C.,for example, GaN layers are crystalline, though slightly defective. Dueto the growth mode at these temperatures being either columnar orthree-dimensional, these layers may need to be slightly thicker toensure complete coverage of the substrate. Slightly defectivecrystalline buffer layers may be grown from about 200 Å to about 1000 Å,for example.

After the nucleation step, the temperatures increased to a hightemperature level to perform a growth step. At these higher temperaturesfrom about 600° C. to about 900° C., for example, amorphous bufferlayers crystallize and crystallinity of the defective crystalline bufferlayers improves. Since thinner amorphous buffer layers crystallize at afaster rate than do thicker amorphous buffer layers, it may be desirableto sustain the temperature in the chamber at a temperature higher thanthe nucleation temperature for a period of time before beginning thegrowth step to ensure the desired crystallization of the nucleationlayer. After the amorphous buffer layer has crystallized, or thedefective crystalline nucleation layer undergoes furtherrecrystallization, any further growth takes place on the crystallizedGaN buffer layer. The films grown by this two step process are superiorto those grown by a one step growth process.

FIG. 2 shows the X-ray diffraction (XRD) pattern of a GaN film grown onthe α-plane of sapphire (11-20) in a one-step process (FIG. 2A) and atwo step process (FIG. 2B). The two peaks at about ca. 2⊖=35° of FIG. 2Aare attributed to a defective GaN crystal. FIG. 2B has a single peakindicating a film of better quality. This is because the majority of thefilm grows on the top of the GaN buffer and does not contact theunderlying substrate. The growth layer of GaN "recognizes" the GaNbuffer layer on which it can grow without defects. The buffer is theonly part of the film which is highly defective. Films grown by themethod described above were highly resistive at room temperature (10¹²ω-cm).

GaN films are doped n-type or p-type by incorporating the properimpurities in their charged state. This is because the energy toincorporate a charged impurity into the lattice is lower than the energyneeded to incorporate a neutral impurity. FIG. 3 is a schematicillustration of an apparatus for incorporating a charged acceptor ordonor into the GaN lattice. The substrate 19 or a grid 19a, directly infront of the substrate 19, is positively biased. FIG. 3 shows bothsubstrate 19 and grid 19a connected to a voltage source. In practice ofthis invention, either substrate 19 or grid 19a may be positivelybiased. Electrons are therefore attracted to the substrate surface,while positive ions such as N⁺ are repelled. The growth process iscarried out as described above with addition of an acceptor source 24 sothat gallium, nitrogen and acceptors are deposited on the electron-richsurface of the substrate. As the acceptor atom approaches the surface,it takes on an electron and is incorporated into the lattice as anegative species. The same procedure is used to dope the GaN latticewith donor impurities, except that a negative bias is used on thesubstrate or the grid. Alternatively, a charged surface can be generatedby bombarding the substrate with electrons or positive ions. Electronguns and ion guns, respectively, are conventional sources of thesespecies.

Suitable acceptor species include, but are not limited to, zinc,magnesium, beryllium, and calcium. Suitable donor species include, butare not limited to, silicon, germanium, oxygen, selenium, and sulfur.

In another embodiment of the present invention, a compact ECR source maybe used. FIG. 4 depicts a compact ECR-assisted MBE device 100 having acompact ECR-system 25 mounted in a Knudsen effusion cell 30. Thus, thissource generates a nitrogen plasma closer to the substrate 19 than aconventional ECR source. Compact ECR-system 25 has an axial solenoid(not shown) to generate the magnetic field used for ECR operation.Compact ECR-system 25 is supplied with nitrogen via nitrogen supply 32.The nitrogen is first purified in nitrogen purifier 28 before enteringcompact ECR-system 25. Compact ECR-system 25 may be an AsTeX compact ECRsource, for example. Compact ECR-system 25 is preferably lightweight,relatively inexpensive compared to traditional ECR sources and operatesat much lower microwave powers than a traditional ECR-source, such asthe AsTeX model 1000 ECR source, for example. Compact ECR-system 25 usesa microwave power for the growth of GaN films in the range of about10-100W. This range of microwave power leads to power densities whichare approximately equal to those resulting from operating traditionalECR sources in the range of about 100-500W.

According to another aspect of this invention, an external solenoid mayalso be used. FIG. 4 depicts an ECR-assisted MBE device 100 with anexternal solenoid 40 attached thereto. However, ECR-assisted MBE device100 may be operated without external solenoid 40 or operated withexternal solenoid 40 having no current. Further, external solenoid 40may also be used with ECR-source 10 depicted in FIGS. 1 and 3, or otheractivated nitrogen sources.

External solenoid 40 is a magnetic device designed to induce a magneticfield. Preferably the magnetic field induced by external solenoid 40 isopposite in sign with respect to substrate 19 to that of the magneticfield generated by the ECR source. External solenoid 40 may be poweredby a dc power supply (not shown), for example, to induce the desiredmagnetic field. External solenoid 40 may comprise a plurality of yurn ofcopper magnet wire wound on a mandrel. For example, 2300 turns of 18gauge enameled copper magnet wire wound on an iron mandrel may be used.The number of turns, type of wire, and construction of the solenoid maybe varied. Preferably, external solenoid 40 is powered by a current ofabout 5-8 amps. Other amperages may also be used, such as from about0-10 amps.

As depicted in FIG. 4, external solenoid 40 is preferably disposedoutside the housing of the high vacuum plasma chamber 15 of MBE system11. External solenoid 40 is preferably disposed along an axis B, whichis preferably disposed at an angle α with respect to an axis A throughECR-system 25. In a preferred embodiment, angle α is about 60°. Othervalues for angle of may also be used. Additionally, external solenoid 40is preferably disposed at a reasonable distance from the magnetic coilin the ECR source to avoid unwanted interference in the ECR source. Inorder for the ECR source to operate properly, a 875 Gauss magnetic fieldis used. If the external solenoid 40 is too close to the magnetic sourcein the ECR source, this field may be effected.

In operation, charged species in ECR plasmas are strongly guided alongmagnetic field lines or, equivalently, down the divergence of themagnetic field by ambipolar diffusion. Thus, the charged nitrogenspecies travel down the compact ECR-system 25 through the effusion port12 along the magnetic field lines generated by the ECR magnetic sourcetoward substrate 19. External solenoid 40 alters the direction of thecharged species (ionic species) which are produced from compactECR-system 25. Further, because of the relatively large separationbetween the external solenoid 40 and the compact ECR-system 25, there isnegligible perturbation of the magnetic field inside the ECR-system 25due to the external solenoid 40 magnetic field. Therefore, the effect onthe species generated in the compact ECR-system 25 is negligible.

FIG. 5 depicts the effects of an external solenoid 40 on the directionof the charged species. FIG. 5A shows magnetic field lines produced froman ECR source without use of an external solenoid 40. As this figureshows, the field lines are symmetrically directed toward substrate 19.FIG. 5B shows magnetic field lines produced form an ECR source with anexternal solenoid 40 in use. The external solenoid 40 serves to divertthe field lines away from the substrate area. Therefore, by varying thepower and position of external solenoid 40 it is possible to selectivelycontrol the amount of and the ratio of ions to excited neutral speciesand atomic species at the substrate. The use of the external solenoidprovides a scalable process. By controlling the effective magneticcurrent of the external solenoid 40, the ion density at the substratemay be altered. Ion density at the substrate may be important becauseions at high energies may cause damage to the growth layer. Therefore,by reducing the amount of ions at the substrate, the likelihood of suchdamage is correspondingly reduced.

FIG. 6 depicts one example of how varying the magnetic current of theexternal solenoid 40 relates to relative ion density. By replacing thesubstrate 19 with the collector of a nude ionization gauge(Bayard-Alpert type) and using it as a Langmuir probe, the relative iondensity as a function of the magnetic current of external solenoid 40may be measured. FIG. 6 illustrates the scalability of the growthprocess through the variation of the magnetic current of the externalsolenoid.

FIG. 7 depicts an I-V characteristic of the Langmuir probe used aboveoperated with external solenoid 40 having a current of 7A and withoutexternal solenoid 40. This figure illustrates that activation ofexternal solenoid 40 results in the reduction of both electron and iondensities at the substrate 19 surface.

Films grown with the external solenoid 40 have improved surfacemorphology, and transport and photoluminescence properties. Thesedifferences are attributed to the reduced number of ions in the nitrogenplasma. By using an external solenoid 40, substantially higher qualityfilms may be grown. This external solenoid thus provides a simple,effective, and unobtrusive method of extracting and controlling chargedspecies, particularly where energetic anisotropies of the ionic speciesresulting from magnetic-field effects make biasing schemes difficult tointerpret.

In another embodiment of the present invention, high energy ion damagemay be reduced through the use of a reduced diameter exit aperture atthe exit of the ECR-system. Compact ECR-system 25 may be provided withan exit aperture 42 at its exit as depicted in FIGS. 4 and 8. Exitaperture 42 may be formed from a disc (not shown) having a hole disposedtherein. The disc may be placed between the ECR system's liner and awafer spring, which holds both the liner and the disc in place. The ECRsystem having the disc at its exit is then placed in effusion port 12.The disc may be made of quartz, for example, and may have a thickness ofabout 1 mm, for example. Other devices for reducing the diameter of theexit of the ECR system may also be used such as a remotely controlledshutter, for example. By using a remotely controlled device, operationof the exit aperture 42 may be varied during operation.

Exit aperture 42 preferably has a diameter, d_(ea), less than thediameter of the ECR-system 25, d_(sys), as shown in FIG. 8. For example,the ECR-system diameter, d_(sys), may be about 2 cm while the diameter,d_(ea), may be varied from about 1 mm to about 1.9 cm, for example,although other diameters may also be used.

By reducing the diameter of the exit of the ECR-system 25, the pressureinside the ECR system 25 is increased. Conversely, increasing thediameter decreases the pressure. Increased pressure promotes collisionsbetween the ions of the plasma. These collisions therefore tend toreduce the energy of the ions and reduce the amount of ionic specieswhich exit the ECR system 25. Any ions that exit the ECR-system 25 havereduced energy and are not as likely to cause damage to the GaN growthlayer. If the diameter of the exit aperture 42 is small enough, no ionsescape due to a screening effect. The diameter at which "screening"occurs is known as the Debrye screening length. Atomic nitrogenparticles, however, pass through this "screen". Atomic nitrogen speciesare preferred for forming GaN layers.

Controlling the diameter of exit aperture 42 allows control of theamount of ionic species and the ionic energy at the substrate. A smallerhole passes fewer ions and ions with reduced energies. The exit aperture42 may be used with or without external solenoid 40. Also, the exitaperture 42 may be used with any type of ECR system or other activatednitrogen source to vary the ionic energy directed at the substrate. Byusing both exit aperture 42 and external solenoid 40, greaterflexibility in controlling the growth process can be provided.

FIGS. 9 and 10 depict the effects of using exit aperture 42. FIG. 9depicts an optical emission spectra for an ECR nitrogen plasma generatedby a compact ECR-system 25 operated at 35 W, with a nitrogen pressure ofabout 1.2×10⁻⁴ Torr without use of an exit aperture. This figure depictsthe presence of ionic nitrogen (N₂ ⁺) at about 391.4 nm, molecularexcited nitrogen (N₂), and atomic nitrogen (N). The energy of the atomicnitrogen (N), occurring at wavelengths higher than about 650 nm, isrelatively small. FIG. 10 depicts an optical emission spectra for an ECRnitrogen plasma generated by a compact ECR-system 25 having a 1 mmdiameter exit aperture 42 in the exit aperture, operated at about 30W.FIG. 10 depicts sharp peaks of atomic nitrogen at about 625 nm and about670 nm. Further, the peak at about 391.4 nm for ionic nitrogen isreduced relative to the size of the atomic nitrogen peaks. As thesefigures indicate, the ratio of energetic ionic species to atomic speciesis reduced through the use of exit aperture 42. Although these figuresdepict results of ECR systems operated at different power levels, an ECRsystem operating with an exit aperture at 35 W would create even greateramounts of atomic nitrogen. Therefore, these figures show the increasein output of atomic nitrogen that is created by increasing the pressurein the ECR system through the use of an exit aperture.

The effects of using the exit aperture 42 are also illustrated in FIG.11. FIG. 11 depicts the ratio of ionic to excited nitrogen species takenfor varying sizes of exit aperture 42 and varying rates of nitrogenflow. Excited nitrogen comprises The squares represent a reading for anECR source operating without an exit aperture. The circles representreadings for an ECR source operating with a 1 cm exit aperture. Thetriangles represent readings for an ECR source operating with a 1 mmexit aperture. As FIG. 11 illustrates, when the diameter of exitaperture 42 is smaller, relative ionic emission intensity is less, evenfor different nitrogen flow rates.

The employment of the external solenoid and/or the use of a restrictingexit aperture in the ECR-system allows the doping of the GaN n- orp-type without the use of the substrate biasing as discussed withreference to FIG. 3. Doping with an external magnet or an reduceddiameter exit aperture may be accomplished by directing a beam ofgallium, activated nitrogen, and the proper dopant toward the substrate.

According to the present invention p-n junction devices such as lightemitting diodes may be formed using various aspects of the inventiondescribed above. One embodiment of a light emitting diode of the presentinvention is depicted in FIG. 12. In FIG. 12, a light emitting diode 150has a substrate 100, a buffer layer 102, an n-type type layer 104, ap-type layer 106, a p-electrode 108 and an n-electrode 110. Substrate100 may be (100) or (111) silicon; (0001) (basal plane), (11-20)(a-plane), or (1-102) (r-plane) sapphire; (111) or (100) galliumarsenide; magnesium oxide; zinc oxide; or (6H-) or (3C-) silicon carbideor other materials that would be conveniently used for epitaxial growthof Group III-V nitride devices. Buffer layer 102 is preferably aGaN-buffer having a thickness in the range of about 100 Å to about 1000Å. A thickness of about 300 Å may be used, for example. As describedabove the thickness may vary depending on the temperature at which it isdeposited.

The n-type layer 104 is epitaxially grown on buffer layer 102 and ispreferably a GaN layer. N-type layer 104 is either autodoped orintentionally doped and has a thickness in the range of about 0.5-10 μm.A thickness of about 1.0-3.0 μm may be used, for example. The p-typelayer 106 is epitaxially grown on n-type layer 104. P-type layer 106 ispreferably a doped GaN p-type layer having a thickness of about 0.3-0.5μm. A thickness of about 0.5 μm may be used, for example.

In a preferred embodiment, as depicted in FIG. 12, p-type layer 106 anda portion of n-type layer 104 may be etched to expose a top surface ofn-type layer 104. N-electrode 110 may be formed on the exposed portionof n-type layer 104 and may comprise titanium and aluminum. Preferably atitanium layer having a thickness of about 200 Å covered by an aluminumlayer having a thickness of about 2000 Å may be used. P-electrode 108may be formed on p-type layer 106. P-electrode 108 may comprise nickel(Ni) and gold (Au). Preferably p-electrode 108 comprises a layer ofnickel having a thickness of about 200 Å and then a layer of gold havinga thickness of about 2000 Å. P-electrode 108 may also comprise platinum.Preferably, the material used for p-electrode 108 should have a workfunction of about 7.5 eV. Platinum has the highest work function of anyknown metal at about 5.8 eV.

In an alternative embodiment, when no etching is performed, p-electrode108 and n-electrode 110 may be formed on p-type layer 106. P-electrode108 contacts p-type layer 106 directly, as in FIG. 12. N-electrode 110contacts n-type layer 104 through p-type layer 106 by soldering withindium metal, for example. Other metals and methods of contacting n-typelayer 104 through p-type layer 106 may also be used.

A method of forming light emitting diode 150 will now be described. Asdescribed above, a substrate 100 is placed in MBE system 11. If thesubstrate 100 is sapphire, then the sapphire preferably is subjected toa nitridation process at a high temperature, 850° C. for example, toform atomically smooth AIN. Other methods of preparing the substratedepending on the substrate used may also be used before GaN growth.

Once the substrate is prepared, the temperature in the MBE chamber isset at between about 100° C. and about 550° C to perform a nucleationstep to grow buffer layer 102 to a desired thickness, for example, 300°Å. As described above, the temperature is one factor in determiningwhether the nucleation layer will be amorphous or defective crystalline.A temperature of about 500° C. may be used, for example. At thistemperature, the activated nitrogen from an ECR-system and the atomicgallium generated by Knudsen effusion cell 22 are directed at thesubstrate 100. Preferably, compact ECR-system 25 is used to generate theactivated nitrogen. The microwave power of the compact ECR-system 25effects the type of growth which is induced. FIG. 13 depicts the effectof microwave power on the surface morphology of films. At 25,000 timesmagnification, FIG. 13(a) depicts the surface of a film grown at 18 W,FIG. 13(b) depicts the surface of a film grown at 20 W, and FIG. 13(c)depicts the surface of a film grown at 25 W. At 18 W, the film has arelatively island-like growth structure. The film grown at 20 W shows arelatively smooth surface typical for layer-by-layer growth. The filmgrown at 25 W shows a three dimensional growth which leads to roughsurface morphologies. A power of about 20 W is preferred to providesmooth layer-by-layer growth as depicted in FIG. 13(b). Other powers maybe used, however, depending upon desired parameters. For example, whenan exit aperture in the ECR-system and/or the external solenoid isemployed, the optimum power in the compact ECR system 25 is betweenabout 30 to about 100 Watts and preferably between about 25-50 Watts.

Then, the substrate is heated gradually at a rate of about 4° C. everyfifteen seconds. Other rates of increasing the temperature may also beused depending on the initial quality of the nucleation layer. Forexample, if the nucleation layer is amorphous, a slower rate ofincreasing the temperature may be used to ensure that the buffer layercrystallizes. The substrate is heated to the desired high temperaturesetting to perform one or more growth steps. A temperature in the rangeof about 600° C. to about 900° C., such as about 800° C., may be used,for example during the growth steps.

Before progressing to the growth step, it is important to determinewhether the buffer layer has begun to crystallize. A reflectionhigh-energy electron diffraction (RHEED) apparatus 46 (FIGS. 1, 3 and 4)may be used to monitor the crystallinity of the nucleation layer. Foramorphous materials, it may be desirable to maintain the temperature atthe high temperature for a period of time, such as at 800 ° C. for about30 minutes, for example, before opening the gallium shutter to begin thegrowth step.

After the desired temperature is established, and the crystallinity ofthe nucleation layer has reached a desired level, the gallium shutter 23is opened to begin growth of the n-type layer 104 on the buffer layer102. The n-type layer 104 is preferably grown to about 1.0-3.0 μm at adeposition rate of about 0.2 μm/hr, for example. As described above,incorporation of n-type impurities may occur by autodoping or byintroduction of a donor impurity. A doping level of between 10¹⁸ -10¹⁹cm⁻³ of net carrier doping may be used.

Once the desired thickness of n-type layer 104 is grown, an acceptordopant is introduced through another effusion cell to form the p-typelayer 106 on top of the n-type layer 104. The acceptor dopant may bechosen from zinc, magnesium, beryllium, and calcium, for example. Otheracceptor dopants may also be used. To facilitate incorporation of theacceptor dopants, the substrate 100 may be biased. When a reduceddiameter exit aperture or an external solenoid are used, however,biasing does not have significant additional benefits to the dopingprocess due to the self-induced substrate biasing which results from theexit aperture and/or the external solenoid.

The Mg-acceptor dopant is preferably sublimated from a conventionalknudsen cell at a lower temperature than the temperature of thesubstrate. A temperature of about 230° C. may be used, for example.During the last few minutes of p-type layer growth, the amount of dopantintroduced into the layer may be increased. In one embodiment, this maybe performed by raising the temperature of the dopant. A temperature ofabout 270° C. may be used, for example. Other methods for increasing thedoping level of the top surface of the p-type layer 106 may also beused. Once the desired thickness of p-type layer 106 is grown, thegallium shutter 23 and the acceptor source are turned off.

By providing the very top layer of the p-type layer with higher dopinglevels, the p-electrode contacts the valence band by electron tunnellingduring LED operation. This is particular useful where the contacts aremade from a metal with a work function less than 7.5 ev. In thissituation, the conduction of electrons from p-electrode to the p-typelayer occurs by electron tunneling.

After the p-type layer 106 has been formed, the device is subjected toetching to remove a portion of the p-type layer 106 and a portion of then-type layer 104 as depicted in FIG. 12, for example. In one embodiment,the etching occurs in a ring-like fashion. During etching, the ring maybe widened to form a hilltop-like formation known as a mesa. Reactiveion etching may be used, for example to remove the desired portion ofthe GaN device of the present invention. Particularly, silicontetrachloride may be used as the etching chemical. Other chlorine basedetching methods may also be used, such as use of Freon-12, for example.Bromide and fluoride based etching may also be used.

P-electrode 108 is then formed on p-type layer 106. P-electrode 108 maybe formed by evaporation of 300 μm diameter dots of 200 Å of nickelfollowed by 2000 Å of gold. As noted, platinum may also be used. Othermetals, diameters and thicknesses for p-electrode 108 may also be used.N-electrode 110 is then formed on n-type layer 104. N-electrode 110 maybe formed by evaporation of a ring around the mesa of 200 Å of titaniumfollowed by 2000 Å of aluminum. A chromium/aluminum ring may also beused. During formation of both p-electrode 108 and n-electrode 106,rapid thermal annealing at relatively high temperatures for a specifiedduration is preferred to form good ohmic contacts. For example, rapidthermal annealing at about 700° C. for about 20 seconds may be used. Insome instances, the n-electrode may be formed, followed by rapid thermalannealing before deposition of the p-electrode.

In another embodiment of a p-n junction device of the present invention,the positions of n-type layer 104 and p-type layer 106 may be reversed.FIG. 14 depicts a light emitting device 160 of the present invention inwhich p-type layer 106 is grown on the buffer layer 102 and n-type layer104 is grown on p-type layer 106. As depicted in FIG. 14, in thisembodiment, p-type layer 106 may be grown to a thickness of about 2.0 μmand n-type layer 108 may be grown to a thickness of about 0.5 μm. Otherthicknesses of these layers may also be used. It is preferred that thebottom doped layer (formed nearest the substrate) be thicker than theother layer. The bottom layer should be less resistive to allow properconduction across the p-n junction. Because resistivity is related tolength and cross-section, the thicker the bottom layer, the lessresistive it is, and correspondingly, the better of a p-n junctiondevice it will make.

In this embodiment, pan of n-type layer 104 and a portion of p-typelayer 106 may be removed by etching. N-electrode 110 then is depositeddirectly on n-type layer 104 and p-electrode 108 is deposited directlyon p-type layer 106.

In yet another method of forming electrodes 108 and 110, the substrate100 and buffer layer 102 may be etched to expose the bottom layer,whether it is the n-type layer or the p-type layer. The correspondingelectrode may then be formed on the bottom layer of the device.

According to another embodiment of the present invention, an n-type GaNsubstrate may be used for substrate 100. In this embodiment, an n-typeGaN layer 104 may be grown directly on the GaN substrate using a singlegrowth step. P-type layer 106 may then be grown on n-type layer 104.P-type layer 106 could also be grown directly on the GaN substrate withthe n-type layer 104 grown on p-type layer 106. Such a structure is ann-p-n bipolar junction transistor. This embodiment may be used incombination with the other structures and methods described except thatas noted, a buffer layer is not necessary. However, if desired, onecould be used.

In another embodiment, a hot tungsten filament or an ion source such asa Kauffman ion source may be used to supply the activated nitrogen. Anion source directs ions in a desired direction. Therefore, unlike theECR source, no magnetic lines are needed to direct the ions. In thesemethods, it may be desirable to bias the substrate positively ornegatively to facilitate the p-and n-type growth respectively, asdiscussed with respect to FIG. 3.

The devices described above may be used to form light emitting diodes orlasers. Advantageously, the use of GaN layers in a p-n junction deviceomits blue-violet light. These devices may be used for a variety ofpurposes including full color displays, optical recording devices, laserprinting, and underwater communication, to name just a few. Thesedevices may also be used to provide semiconductor laser devices.

EXAMPLE 1

One example of a light emitting diode having a p-n junction according tothe present invention is described below. This example is not intendedto limit the invention.

A (0001) sapphire substrate was first subjected to a nitridation processat 850° C. to convert its surface to atomically smooth A1N. Next, aGaN-buffer approximately 300 Å thick was deposited at about 500° C.Following the deposition of the GaN-buffer, the substage was heated toabout 800° C. and an autodoped n-type GaN film approximately 2 μm wasdeposited at a deposition rate of 0.2 μm/hr. Then, a Mg-doped p-layerapproximately 0.5 μm thick was deposited by incorporating Mg which wassublimated from a conventional Knudsen cell at 230° C. The Mg flux wasthen gradually raised by about an order of magnitude by increasing thecell temperature to about 270° C. near the end of the run to facilitatethe electrical contacting of the top p-layer. The device was grown witha reduced diameter aperture of about 1 cm at the exit of the ECR source.

The top p-layer was electrically contacted by thermal evaporation of 300μm. diameter dots of 200 Å of nickel followed by 2000 Å of gold. Thebottom n-layer was contacted through the p-layer be soldering withindium metal. For testing purposes, the chip was then mounted in a chipcarrier with silver paint and connected with an ultrasonic wire bonder.

A second device was grown using the above procedure additionally usingan external solenoid operating at a magnet current of about 7 amps.

Results

The n-type layers had a carrier concentration of about 5×10¹⁸ cm-³ andelectron mobilities of about 80 cm² /Vs. The p-type layers had a netcarrier concentration of 5×10¹⁷ cm³ and a hole mobility of 6 cm² /Vs.The I-V characteristics of the devices indicated a turn on voltage ofaround 3 volts.

The devices' emission spectra were measured by dispersing the lightthrough a spectrometer and detecting it with a photomultiplier. Thedevices were driven with a pulse generator at about 40 Hz and a 10% dutycycle to facilitate locked-in measurements. The spectra were measured ata drive current of about 150 mA or a current density of about 212A/cm-².

The EL spectra for the above devices were measured after immersing thechip in liquid nitrogen at about 77K. FIG. 15 depicts the results of theEL spectra measurements. Curve (a) represents the EL spectra of a devicefabricated from layers grown without external solenoid 40 and curve (b)represents the EL spectra of a device fabricated from layers grown withexternal solenoid 40 driven at about 7 amps. The peak emission at about430 nm is characteristic of Mg-doped GaN homojunction LED's. This peakis in the violet part of the spectrum. By using external solenoid 40 tominimize ion damage in the film, a reduction in this tail as well as anenhancement of the peak at about 430 nm is achieved resulting in abluish-violet apparent color.

A GaN blue-violet light emitting p-n junction has thus been grown byMBE. These devices do not require a post-growth thermal annealing orLEEBI treatment step to activate the Mg acceptors in the p-layer. Blueemissions characteristic of Mg luminescence centers are observed.

EXAMPLE 2

An LED device was grown using a procedure as set out in Example 1 withan external solenoid. Instead of placing the contacts on the p-typelayer, reactive ion etching was performed to expose the underlyingn-type layer. A p-electrode and an n-electrode were then formed on thep-type layer and the n-type layer, respectively, followed by rapidthermal annealing of the metal contacts as described above.

Results

The I-V characteristics of a device as described in Example 2 areillustrated in FIG. 16. The device of Example 2 exhibits excellent I-Vcharacteristics with a turn on voltage of about 3 volts.

While this invention has been described with reference to specificembodiments, it is not intended that the invention been limited thereto.The invention is only limited by the claims which follow.

What is claimed is:
 1. An epitaxial growth system comprising:anepitaxial growth chamber; a substrate support located within the growthchamber for supporting a substrate; a gallium source for introducinggallium into the growth chamber and directing gallium towards thesubstrate; a plasma assisted nitrogen source for introducing activatednitrogen species into the growth chamber and directing the activatednitrogen species towards the substrate, the activated nitrogen speciescomprising ionic nitrogen species and neutral excited species, whereinthe neutral excited species comprise atomic nitrogen species and excitedmolecular species, the plasma assisted nitrogen source comprising anexit aperture through which the activated nitrogen species pass; andmeans for controlling the ratio of the neutral excited species versusthe ionic nitrogen species, said means for controlling the ratio of theneutral excited species versus the ionic nitrogen species comprisingmeans for varying the diameter of the exit aperture of the plasmaassisted nitrogen source.
 2. The epitaxial growth system of claim 1wherein the means for controlling comprises means for biasing thesubstrate.
 3. The epitaxial growth system of claim 1 wherein the meansfor controlling comprises a magnetic device.
 4. The epitaxial growthsystem of claim 3 wherein the magnetic device is disposed outside of thegrowth chamber.
 5. The epitaxial growth system of claim 3 wherein themagnetic device comprises a solenoid disposed outside of the growthchamber.
 6. The epitaxial growth system of claim 3 wherein the magneticdevice modifies the magnetic environment in front of the substrate. 7.The epitaxial growth system of claim 3 wherein the magnetic devicemodifies the magnetic environment in front of the substrate to vary theratio of neutral excited species to ionic nitrogen species withoutsubstantially modifying the magnetic field within the activated nitrogensource.
 8. The epitaxial growth system of claim 1 wherein said plasmaassisted nitrogen source comprises a microwave plasma assisted ECRsource, and the means for varying the diameter of the exit aperture ofthe plasma assisted nitrogen source comprises means for controlling thepressure inside the microwave plasma assisted ECR source.
 9. Theepitaxial growth system of claim 1 further comprising means forcontrolling the energy of the ionic nitrogen species.
 10. The epitaxialgrowth system of claim 9, wherein the means for controlling the energycomprises an exit aperture at the exit of the activated nitrogen source.11. The epitaxial growth system of claim 9 wherein the means forcontrolling the energy comprises a magnetic device which controls theenergy of the ionic nitrogen species reaching the substrate.